Coding of stereoscopic depth information in visual areas V3 and V3A.

Department of Brain and Cognitive Sciences, Center for Visual Science, University of Rochester, Rochester, New York 14627, USA.

Abstract

The process of stereoscopic depth perception is thought to begin with the analysis of absolute binocular disparity, the difference in position of corresponding features in the left and right eye images with respect to the points of fixation. Our sensitivity to depth, however, is greater when depth judgments are based on relative disparity, the difference between two absolute disparities, compared to when they are based on absolute disparity. Therefore, the visual system is thought to compute relative disparities for fine depth discrimination. Functional magnetic resonance imaging studies in humans and monkeys have suggested that visual areas V3 and V3A may be specialized for stereoscopic depth processing based on relative disparities. In this study, we measured absolute and relative disparity-tuning of neurons in V3 and V3A of alert fixating monkeys, and we compared their basic tuning properties with those published previously for other visual areas. We found that neurons in V3 and V3A predominantly encode absolute, not relative, disparities. We also found that basic parameters of disparity-tuning in V3 and V3A are similar to those from other extrastriate visual areas. Finally, by comparing single-unit activity with multi-unit activity measured at the same recording site, we demonstrate that neurons with similar disparity selectivity are clustered in both V3 and V3A. We conclude that areas V3 and V3A are not particularly specialized for processing stereoscopic depth information compared to other early visual areas, at least with respect to the tuning properties that we have examined.

Horizontal disparity tuning curves for 6 example neurons from area V3 (A–F) and 6 neurons fromV3A (G–L). In each case, disparity of the center patch was varied while the surround disparity remained 0 (fixation plane). Solid lines are the Gabor functions that best fit the data. Horizontal dashed lines indicate spontaneous activity levels. Each error bar indicates the standard error of the mean.

Population distributions of preferred disparity (A–C) and Gabor phase (D–F). A. The distribution of preferred disparities for V3 neurons. B. The distribution of preferred disparities for V3A neurons. C. Cumulative distributions of preferred disparities for V3 (red) and V3A (cyan) neurons and corresponding data for V1 (blue), V4 (pink), and MT (green) neurons, which are replotted from , , and , respectively. Dashed lines indicate distributions for neurons with eccentricities less than 10°. D. The distribution of Gabor phase for V3 neurons. E. The distribution of Gabor phase for V3A neurons. F. Distributions of Gabor phase for V3 and V3A neurons, together with data for V1, V4, and MT neurons that are replotted from the previous studies cited above except for V1 data, which are from . Color conventions as in panel C.

Disparity frequency as a function of eccentricity for V3 (red diamonds) and V3A (cyan circles) neurons. For comparison, data for V1 (blue triangles) and MT (green squares) neurons are replotted from and , respectively.

Comparison of DDI values for pairs of disparity tuning curves measured at different surround disparities. Black circles: cases where disparity tuning is significant for both surround disparities; gray triangles: cases where tuning is significant for one of the two surround disparities; white squares: cases where neither surround disparity has significant tuning. A. Data from area V3. B. Data from V3A.